Lactic acid is used as a food acidulant and flavoring and in pharmaceuticals, plastics, textiles and other industrial formulations. The increased use of food and pharmaceutical products formulated with lactic acid has been primarily responsible for growth of world wide production of lactic acid to about 300 million pounds per year which is expected to continue in the future.
Lactic acid is produced by a submerged culture fermentation process which employs molasses, potatoes or starch as feed and a microorganism such as Lactobacillus del brueckii, L. bulgarcius, or L. leichnanii. The fermentation product also contains carbohydrates, amino acids, proteins and salts in addition to lactic acid, necessitating a more or less elaborate separation and purification scheme.
In the customary separation of lactic acid, the calcium salt is precipitated. The resulting calcium lactate is filtered to remove heavy metal and some organic impurities. Lactic acid is subsequently regenerated using H.sub.2 SO.sub.4 and is separated from the precipitated CaSO .sub.4, as by filtration, and the resulting crude lactic acid is then further purified by carbon treatment and sodium ferrocyanide to remove additional organic impurities and heavy metals, respectively. After filtration, the lactic acid is contacted with an ion exchange resin to remove trace ions. The purification process is complex and high purity is difficult to obtain.
More recently lactic acid has been used as a monomer in the preparation of, for example, poly(lactic acid) and copolymers such as those with glycollic and methylglycollic acid, polymers which are of increasing interest because of their biodegradability. For example, polymers and copolymers of lactic acid have been used as microencapsulants for the controlled release of pharmaceutically active agents; as the polymer degrades it effectuates a controlled release of the encapsulated drag. A drug delivery device even can be surgically implanted where the device is designed to release the drug slowly over extended periods of time. Polymers and copolymers of lactic acid also are used medically as, for example, sutures and wound closing staples.
The medical uses of the polymers and copolymers of lactic acid require high monomer purity. But in addition to the medical uses of lactic acid polymers and copolymers there is widespread interest in the use of these polymers and copolymers as commercial biodegradable polymers in, for example, containers for the fast food industry. Although the purity of monomeric lactic acid in the strictest sense of the word may not be of prime importance for commercial products in non-medical applications, the appearance of the polymeric product is of great importance. In particular, it is important that commercial polymers not be discolored. This often is a problem because polymers are prepared at high temperatures (approximately 200.degree. C.) where commercial lactic acid frequently shows low heat stability in the sense of developing color bodies. In fact, for the preparation of a colorless lactic acid polymer or copolymer it is necessary that lactic acid remain colorless upon being heated at 180.degree. C. for 3 hours. Even where the lactic acid is subjected to more or less elaborate purification schemes the resulting purified lactic acid still may not exhibit the requisite heat stability and hence may be unsuitable in the preparation of lactic acid polymers and copolymers.
Confronted with the problem of producing a heat-stable lactic acid, we noted that lactic acid produced via fermentation almost invariably developed color when heated, regardless of the purification methods used. This rather remarkable observation of susceptibility to color development being independent of the purification process applied led us to surmise that color development was associated with trace amounts of carbohydrates in the lactic acid sample which underwent carbonization upon heating. Were this hypothesis correct, then a heat-stable lactic acid could be readily prepared by subjecting an aqueous solution of lactic acid to a sufficient heat treatment to develop color bodies via carbonization of carbohydrates with subsequent removal of the color bodies. Since no uncarbonized carbohydrates would remain at this point, lactic acid subsequently isolated would manifest the requisite heat stability. In fact, this hypothesis proved correct. What was particularly gratifying was the observation that heat stability did not require that the heat treatment be applied at a particular stage in lactic acid purification, but rather that heat treatment could be effected at various stages in the purification process. This is not to say that all variants are equally effective, but rather that there is a wide range of options within a general lactic acid purification scheme where heat treatment can be incorporated ultimately to afford a heat-stable lactic acid.